docs: def print_structure(self, indent=0, tostring=False):
          """
          For debugging.
          """
          nl = '\n'
          sp = indent * '    '
  
          rlist = [str(self) + nl]
          if self.s_center:
              for iv in sorted(self.s_center):
                  rlist.append(sp + ' ' + repr(iv) + nl)
          if self.left_node:
              rlist.append(sp + '<:  ')  # no CR
              rlist.append(self.left_node.print_structure(indent + 1, True))
          if self.right_node:
              rlist.append(sp + '>:  ')  # no CR
              rlist.append(self.right_node.print_structure(indent + 1, True))
          result = ''.join(rlist)
          if tostring:
              return result
          else:
              print(result)